Many modern television systems include provisions for displaying an auxiliary image corresponding to an auxiliary video signal source as well as a main image corresponding to a main video signal source. Such provisions allow a viewer to monitor one program while watching another program. The auxiliary image may be displayed within the main image in a so called "picture-in-picture" or "PIP" format. The auxiliary image may also be displayed next to the main image in a so called "picture-outside-picture" or "POP" format.
The auxiliary image is formed by converting the analog video signal from the auxiliary signal source to a digital video signal, "subsampling" the digital video signal by eliminating pixel samples and lines from the digital video signal, storing the resulting compressed digital video signal in a memory, and retrieving the stored digital from the memory in synchronism with the horizontal and vertical synchronizing components of the video signal from the main video signal source. A so-called "antialiasing" filter is utilized to reduce the bandwidth of the digital signal before compression to avoid artifacts in the auxiliary image due to the subsampling process. The digital signal retrieved from the memory is converted to an analog video signal and the resultant analog video signal corresponding to the auxiliary image is combined with the analog signal corresponding to the main image to form a combined analog video signal. The combined analog video signal is coupled to a display device. The combining operation is performed by a switching section or "multiplexer" which selectively decouples the main video signal from the display device and instead couples the auxiliary video signal to the display device during predetermined portions of predetermined horizontal scanning lines of the main video signal.
The auxiliary image is usually considerably smaller than the main image. For example, the auxiliary image may be one-third the size of the main image. However, it may be desirable to be able to selectively reduce the size of the auxiliary image still further, for example, from one-third size to one-fourth size, so that the auxiliary image does not obscure the main image. One way of accomplishing this is to change the compression ratio of the auxiliary image. However, this requires relatively extensive and complicated circuitry for changing the subsampling ratio and the associated anti-aliasing filtering, and changing the management of the memory for storing of the subsampled digital signal. Moreover, the additional compression tends to make details of the auxiliary image and text and graphics inserted within the auxiliary image harder to see.